Pacemakers have long been used to stimulate the proper operation of patients' hearts. In a healthy heart, the sinoatrial node produces periodic electrical pulses that travel downward across the atria and ventricles. The spread of each pulse depolarizes the muscle fibers and, after the pulse passes, the fibers contract, repolarize, and return to their resting state. The operation of the heart in this manner pumps blood throughout the cardiovascular system.
The ability of the sinoatrial node to successfully fulfill its obligations as the heart's natural pacemaker can, however, be impaired by coronary disease and injury. In such instances, it may be necessary to apply artificially generated electrical pulses to the heart to stimulate proper operation. These pulses are generally produced by an implantable or external pacemaker.
Reviewing, for example, the construction and operation of an external pacemaker in greater detail, a pair of pacing electrodes is applied to the patient's chest. These electrodes may be positioned in an anterior/anterior configuration or an anterior/posterior configuration as desired. The electrodes are connected to the remotely positioned pacemaker by cables.
The pacemaker typically includes a number of controls that allow medical personnel to regulate the operation of the pacemaker. In that regard, if the pacemaker is part of a more comprehensive system that includes, for example, an electrocardiogram (ECG) monitor and defibrillator, a button or switch for selecting the pacing mode of operation is typically included.
In addition, most external pacemakers include a button or switch that allows the operator to select between two different modes of pacing operation: continuous and demand. In the continuous mode of operation, pacing signals are continuously applied to the patient until pacing is manually disrupted by the operator. In the demand mode of operation, the patient's ECG signals are monitored and the pacemaker determines when pacing is to be performed.
The pacemaker also usually includes controls that allow the operator to regulate the nature of the pacing signals applied to the patient. In that regard, a button or switch may be included to allow the operator to adjust the rate at which electrical pulses are applied to the patient. Further, the pacemaker may include a button or switch that the operator can use to adjust the magnitude of the pacing current applied to the patient. In that regard, the magnitude selected will usually be the lowest one that is sufficient to "capture" the heart, or restore it to proper operation. Finally, a START/STOP button is generally included to initiate pacing after the various other inputs have been provided.
One example of a conventional pacing system is provided by the LIFEPAK 10 external pacemaker/monitor/defibrillator, available from Physio-Control Corporation, the assignee of the present application. Among the various controls associated with this product is a "PACEMAKER" pushbutton that enables the pacing portion of the instrument.
A toggled, "RATE" pushbutton allows the rate at which electrical pulses are produced to be input by the operator. More particularly, the operator can increment the pacing rate by depressing one side of the pushbutton and can decrement the pacing rate by depressing the other side of the pushbutton. The product is microprocessor based and responds to actuations of the RATE pushbutton in accordance with software instructions programmed into memory to achieve the desired rate regulation.
The LIFEPAK 10 product also includes four pushbuttons that allow the operator to adjust the magnitude of the pacing current. For example, the operator can increase or decrease the pacing current in five-milliampere increments by depressing separate "5" milliampere pushbuttons. Similarly, separate "20" milliampere pushbuttons are provided to allow the pacing current to be increased and decreased in 20-milliampere increments.
As with the rate control pushbutton, the instrument's microprocessor responds to actuations of the four current control pushbuttons in accordance with software instructions programmed into the instrument. In addition, the software instructs the microprocessor to monitor the operation of, for example, the PACEMAKER and START/STOP buttons to automatically reset the pacing current to zero in the event either switch is actuated. As a result, at startup, the pacing current defaults to zero, regardless of the last setting established with the four pacing current pushbuttons.
The inclusion of this reset feature ensures that the initial application of a nonzero pacing current to a patient is the result of a specific operation action. Such a limitation is important, in part, because the unintentional application of an unduly high pacing current level may needlessly increase pain or discomfort experienced by the patient as a result of the pacing operation.
As an alternative to the use of pushbutton switches of the type described above, potentiometers have also been used to allow an operator to regulate the magnitude of the pacing current applied. One example of a defibrillator/monitor/pacemaker that employs a potentiometer for pacing current control is the LIFEPAK 8 product sold by Physio-Control Corporation, the assignee of the present application. The use of such a rotary device has the advantage of allowing an operator to adjust the pacing current level more quickly than can conventionally be achieved with pushbutton switches.
The conventional potentiometer used in the LIFEPAK 8 product, however, has a limited range of rotation, with each rotational position having a particular resistance and, hence, pacing current associated therewith. As a result, the LIFEPAK 8 defibrillator/monitor/pacemaker produces a zero pacing current upon startup only if the potentiometer is manually rotated to its zero position before pacing is initiated. In emergency situations, this manual step can easily be forgotten.
The LIFEPAK 8 and LIFEPAK 10 products discussed above both employ a mechanism for selecting the pacing operation, as well as a pacing START/STOP button. As suggested above, the inclusion of a separate START/STOP pushbutton requires the operator to actively initiate pacing. Thus, pacing is unlikely to begin at an inadvertently set rate or current level. The inclusion of the START/STOP pushbutton on the defibrillator/monitor/pacemaker does, however, contribute to the complexity of the instrument's control panel.
In view of these observations, it would be desirable to produce a pacemaker control system that allows an operator to quickly and precisely control the magnitude of the pacing current applied. The control system should also ensure that the pacing current is automatically reset to zero at the initiation of pacing. Finally, the inputs to the pacemaker control system should preferably be limited to the minimum required for proper operation.